Battle of Mt. Jupil

The Battle of Mt. Jupil was fought in 645 AD between the invading Tang Chinese army of Emperor Taizong of Tang and the defending Goguryeo Korean army of Yeon Gaesomun. The Tang army launched a cavalry attack on the Goguryeo rear as a Goguryeo attack seemingly made progress, massacring the Koreans and reducing an army of 150,000 Goguryeo troops to a mere 10,000.

Background
Emperor Taizong of Tang was nicknamed the "God of War", and, in 645 AD, he invaded the Korean kingdom of Goguryeo and instantly captured many fortresses along the border. The Goguryeo general Yeon Gaesomun dispatched 150,000 troops to face Taizong at Mt. Jupil, the first battle of the war.

Battle
The 150,000-strong Korean army began the battle with a massive cavalry charge against the 200,000 defending Chinese troops. The Goguryeo army's morale was high, and their shock cavalry broke through the Chinese ranks as they attempted to form a defensive spear wall. Just as the Goguryeo forces threatened to annihilate the main Chinese force, a Chinese cavalry force charged around the Korean rear and slaughtered them from behind. Taizong then ordered a general counterattack which routed the Korean army, of which only 10,000 survived; the rest died in battle. Emperor Taizong now had only to take Ansi fortress before he could march on the Goguryeo capital.